1. Field of the Invention
The present invention relates to an image forming apparatus such as copying machines, facsimiles, printers or their combination and particularly an image forming apparatus comprising magnification changing means for changing the size of an image to a selected size and automatic density regulating means for automatically or semi-automatically controlling the density of the image.
2. Description of the Prior Art
Recently, a mechanism for automatically regulating the density of an image has been incorporated into some image forming apparatus which forms an image from an original or in accordance with signals, such as copying machines, facsimiles or printers.
For example, U.S. Pat. No. 4,215,930 discloses a copying machine in which the reflected light from an original is received by a photosensor to detect the density of the background of the original and control the development bias in accordance therewith.
On the other hand, electrophotographic systems have currently be widely utilized as copying machines. Some of the electrophotographic systems are provided with a magnification changing mechanism for permitting an image formation from an original in the desired size, that is, one-to-one size, an enlarged size or a reduced size.
In general, the automatic density regulating mechanism is adapted to process the density of an image to be formed in accordance with a predetermined manner. There will, therefore, be a difference in density between images formed in a changed magnification and the unit magnification. Such an automatic density regulating mechanism sometimes cannot function to obtain a proper image.
There will now be described the key section of one example of the conventional automatic density regulating systems.
In the example of the prior art automatic density regulating systems used in copying machines or the like, an original is scanned prior to the actual copying step to measure potentials in a latent image formed in a photosensitive member and integrate the measurements over the range of scan. The integration is then used as a detection of the density of the image on the original.
Referring to FIG. 1, prior to the actual copying step, an optical system is moved from the start point P to the end point Q in the direction of arrow A to illuminate the original with the light from a halogen lamp 7 in the optical system such that a latent image of the original O will be formed on a photosensitive drum 1. Potentials on the latent image are measured by a potential sensor 2, and the measurements are integrated by an integrating circuit 4. The integration is taken as the density of the original. This integration is one obtained by integrating the potentials with time required to move the optical system from the point P to the point Q. If the speed of the optical system is changed to vary the magnification in which an image is to be formed, however, the time required to move the optical system from the point P to the point Q also is naturally changed to provide a different integration. For example, if it is assumed that the time required to move the optical system from the point P to the point Q is t and the time constants is I in the one-to-one copying mode (unit magnification), the integration is 100{1-e-(t/I)} (V). If the optical system is moved at a speed 1/0.7 times that of the one-to-one copying mode to vary the magnification to a reduction of 0.7, the integration time is equal to 0.7 t. If this time is used to integrate the constant potential of 100 V on a latent image of a half-tone original having the same density, the resulting integration becomes 100{1-e-(0.7t/I)} (V). In other words, the same original may be judged to have different densities if the optical system is moved at different speeds. If a copying step is executed controlling the amount of light without correcting the erroneous density measurement, a proper copy will not be obtained when the magnification is changed.
As long as one of the components of the optical system including an original carriage and optical parts such as mirrors, lamps and others is adapted to move at a different speed if the magnification is changed from one to another, a proper image could not always be obtained even if the density of an original was detected by other types of conventional means.
It is thus desired to provide a suitable system for effectively overcoming such a problem in the conventional automatic density regulating mechanism and particularly in an apparatus including means for moving a movable image forming member at different speeds matching the selected magnifications.